The present invention relates to a process for doping molten silicon for use in a crystal growing process. More particularly, the present invention relates to a process for doping molten silicon with barium for use in combination with a silica crucible containing a very low level of gases insoluble in silicon and/or one or more tungsten doped layers such that the barium dopant causes a thin devitrified layer of silica to form on the inside crucible walls during melting of the polysilicon and throughout the ingot growing process without significant barium dopant incorporation in the growing ingot.
In the production of single silicon crystals grown by the Czochralski method, polycrystalline silicon is first melted within a quartz crucible. After the polycrystalline silicon has melted and the temperature equilibrated, a seed crystal is dipped into the melt and subsequently extracted to form a single crystal silicon ingot while the quartz crucible is rotated. Due to the extremely high temperatures achieved during ingot growth, the quartz crucible walls are slowly dissolved at the crucible-melt interface as the ingot is grown. One disadvantage associated with the use of vitreous silica crucibles is the fact that contaminants on the inner surface of the crucible can nucleate and promote the formation of cristobalite islands in the vitreous silica surface (the islands being centered, in general, about the contamination site) as the polysilicon is melted and the single crystal ingot is grown. The cristobalite islands can be undercut during melting and released as particles into the silicon melt, causing the formation of dislocations in the silicon ingot. The cristobalite islands can be undercut, for example, by the action of a low melting eutectic liquid formed at the interface between the vitreous silica and cristobalite as described by Liu et al., "Reaction Between Liquid Silicon and Vitreous Silica," J. Mater. Res., 7(2), p. 352 (1992).
To reduce the amount of contaminants released into the melt, silica crucibles used in crystal growth generally have two distinct zones. The outer zone of the crucible in contact with the graphite mechanism supporting the crucible contains a high density of bubbles to regulate radiant heat transfer to the melt and crystal. The inner zone contains a reduced bubble layer commonly referred to as a clear layer or bubble free layer. This inner layer is not totally bubble free and upon exposure to temperatures typical to crystal growth dissolved or trapped gas near the crucible surface can form bubbles on the crucible surface and be released into the silicon melt. the release of bubbles over an extended period of time can cause degradation of the inner layer of the crucible and voids in the growing ingot. This degradation is a time limiting factor for crystal growth and can result in a loss of zero dislocation structure or physical defects such as large light point defects in the grown crystal.
Various approaches are known in the art for reducing contaminant production by improving the durability of the inner layer of the crucible through either stabilizing the silica/silicon interface or increasing bubble stability within the crucible surface. Some of the approaches include improving the stability by controlling hydroxide content of the inner layer below a certain value (Matsumura Japanese Patent Application 08-169798), forming a bi-layer structure by fusing a preformed silica tube (the inner layer) into a backing layer (bubble composite) (Watanabe et al. Japanese Patent Application 08-333124), and annealing the crucible in a hydrogen atmosphere at elevated pressure to incorporate hydrogen into the silica such that upon exposure to the melt and the subsequent dissolution of the silica, hydrogen is incorporated into the silicon crystal to reduce stacking faults.
Additionally, others have attempted to reduce or eliminate contaminant production by the crucible into the melt and/or crystal by improving the durability of the silica through the use of a devitrification promoter coating pre-applied to the crucible surface prior to the introduction and heating of polysilicon (Hansen et al. EP 0748885A1, EP 0753605A1.) Upon the melting of the polysilicon, these coatings cause a devitrified silica surface to form in the presence of the silicon melt throughout the crystal pulling process.
Although several attempts have been made to improve crucible performance and reduce contamination of the melt during the ingot growing process, none of the attempts to date have been completely successful in eliminating all contaminant production by the crucible. As such, a need still remains in the art for an improved crucible capable of producing ingots with reduced contaminant and defect levels.